1. Field of the Invention
This invention relates to the field of ceramic composites which comprise a continuous phase, also interchangeably called matrix, and a discontinuous phase, also interchangeably called reinforcement. The discontinuous phase is at least predominantly in the form of elongated fibers. Such materials are generally denoted in the art as fiber reinforced composites. This invention relates more particularly to composites with a matrix comprised predominantly of silicon nitride and reinforcing fibers predominantly of silicon carbide.
2. Technical Background
Like almost all other ceramics, silicon nitride inherently has little ductility, extensibility, or other capacity for stress relief, so that when subjected for even a short time to mechanical stresses in excess of its capacity, it normally breaks. Practical uses for ceramic objects generally expose them to discontinuous and non-uniform mechanical loads, so that the mechanical stress in small areas of the ceramic can easily exceed the capacity of the ceramic even when the overall stress is well below a value which would lead to fracture in laboratory testing. High stresses in small areas cause cracks to form, and because cracks themselves concentrate stress at their tips, a single initial crack can propagate entirely across a ceramic object, causing its catastrophic failure.
Although the term "catastrophic" is often used loosely to describe the failure of materials, for purposes of this application it is useful to give it a more precise definition, with reference to a conventional measurement of the stress induced in a material by mechanical strain. For most materials, including ceramics, the relation between stress and strain is linear at low strains. Increased strain leads eventually to a value, called the yield strain, at which the rate of increase of stress with increasing strain begins to fall below the value it had at very low strains. For typical unreinforced ceramics, the yield strain coincides with fracture of the ceramic, so that the stress falls essentially to zero. Failure of a body is defined as catastrophic for purposes of this application if the stress on the body at a strain 10% higher than the yield strain is less than 20% of the stress on the body at a strain 2% less than the yield strain.
A method well known in general terms in the art for improving the mechanical stability of typically brittle ceramics such as silicon nitride is reinforcing the ceramic with inclusions of other material, often another ceramic. Small ceramic fibers or other particles, because of more nearly perfect crystallinity, are usually stronger and sometimes more shock resistant than bulk bodies, even of the same nominal ceramic composition, which are made by conventional practical processes such as powder sintering or reaction bonding. Reinforcement, of course, need not be limited to particles of the same composition as the matrix, and often it is advantageous to utilize a different composition for some particular property in which it is superior to the matrix.
In some but far from all cases, reinforcement, especially with elongated strong fibers, will prevent catastrophic fracture of a composite, even under conditions expected to cause fracture of the matrix of the composite alone. This improvement in fracture resistance from fiber reinforcement is believed to result primarily from three mechanisms generally recognized in general terms in the prior art: load transfer, crack bridging, and debonding.
Until the recent past, most new types of fiber reinforced composites were made by workers trying to improve strength or rigidity. For such purposes a strong bond between the matrix and the reinforcing fibers is needed, so that strong bonding has usually been a goal. For example, an improvement in modulus of rupture for composites having a silicon nitride matrix formed by sintering was disclosed by Yajima et al. in U.S. Pat. No. 4,158,687. Continuous silicon carbide fibers formed by a special process described in U.S. Pat. No. 4,100,233 were used as the reinforcement, and "polycarbosilane" powder was added to the silicon nitride powder to improve the bonding between the matrix and the fibers. By these means a composite body containing unidirectionally oriented fibers with a modulus of rupture (denominated in this instance as "flexural strength") of 610 MPa was achieved. Good oxidation resistance, corrosion resistance, heat resistance, and strength at high temperatures were asserted as properties of the composites formed, but nothing was stated about the nature of the rupture of the composite.
In fiber reinforced composites with such tight bonding as illustrated by this Yajima patent, cracks resulting from concentrated mechanical stresses in the matrix tend to propagate into the fibers and crack them as well. Recent workers have discovered that such undesirable crack propagation can be avoided by surrounding the reinforcing fibers with a crack deflection zone. The crack deflection zone should have mechanical properties which will cause most cracks which propagate into the zone from the matrix either to be arrested or to follow a path which will keep them away from the reinforcing fibers.
One of the earlier workers to recognize the possible value of coating fibers with weakly bonded coatings appears to have been Warren, as exemplified by EPO Application 0 121 797 published Oct. 17, 1984. On page 4 lines 23-25, this application states, "[P]oor fiber to matrix bonds produce tough composites while good fiber to matrix bonds result in brittle, flaw sensitive materials." In the embodiment believed most relevant to the present application, the Warren application teaches forming an array of carbon fibers, coating them while they are in the array with a layer of pyrolitic carbon, machining the resulting porous body to the desired final shape, overcoating with a layer of chemically vapor desposited silicon carbide, heating to about 2700.degree. F. "to effect dimensional stability between the silicon carbide/pyrolitic carbon and the substrate", and finally overcoating again with a chemically vapor deposited silicon nitride.
Because the carbon fibers which formed the original substrate were arrayed in a fabric, felt, or similar structure before coating, the coating was not uniform around the fibers, as clearly illustrated in FIG. 3 of the Warren application drawings: where two of the original fibers touched in the original array, the coating apparently could not penetrate between them.
The pyrolytic carbon layer deposited according to the teachings of Warren was so weakly bonded that the "fibers were free to move at a different rate from the carbon and/or silicon carbide matrix systems."
Because of its low coefficient of thermal expansion, silicon nitride has long been regarded as one of the most attractive ceramics for use in conditions requiring resistance to thermal stresses. Nevertheless, the low mechanical shock resistance of unreinforced silicon carbide at almost any practical service temperatures and its low creep strength at high temperatures seriously limit its practical uses.
One of the early attempts to improve the properties of silicon nitride by inclusion of other materials in it was disclosed by Parr et al. in U.S. Pat. No. 3,222,438. This taught the inclusion of 5-10% of silicon carbide powder among silicon metal powder which was to be converted to a solid ceramic body by treatment with nitrogen gas at a sufficiently high temperature to promote the conversion of the silicon to its nitride. This process, termed reaction bonding, produced coherent silicon nitride ceramic bodies with creep resistance significantly improved over those made without the silicon carbide powder additions. The bodies to be fired were formed from powders by cold pressing in a die set, and the addition of cetyl alcohol as a binder and lubricant for the powder before pressing was recommended. The disclosure of this patent strongly recommended, and the claims all required, that the reaction bonding temperature exceed 1420.degree. C., the melting point of silicon, during part of the bonding cycle. The modulus of rupture for the composite bodies formed was not given, being described merely as comparing "favourably [sic] with those already published by others".
The use of relatively short silicon carbide fibers for reinforcing ceramics was disclosed by Hough in U.S. Pat. No. 3,462,340. Orientation of the fibers by mechanical or electrostatic forces was taught as an advantage in this patent, but no quantitative information about the mechanical properties of the resulting composites was given. Moreover, the matrix of the composites taught by this patent was limited to "pyrolitic" materials. The term "pyrolitic" was not particularly clearly defined in the patent specification, but it was apparently restricted to materials having all their chemical constituent elements derived from a gas phase in contact with the hot reinforcing filaments and a mold-like substrate which determined the inner shape of the body to be formed. No method was taught or suggested in the patent for obtaining silicon nitride as a "pyrolitic" product within this definition.
A use of very short fibers of silicon carbide to reinforce ceramic composites having a silicon nitride matrix was taught by Komeya et al. in U.S. Pat. No. 3,833,389. According to the teachings of this patent, the matrix was formed by sintering silicon nitride powder rather than by nitriding silicon metal powder, and the maximum length of the silicon carbide fiber inclusions was 40 microns. A rare earth component was required in the matrix in addition to silicon nitride, and the highest modulus of rupture (denominated as "breaking strength") was 375 megapascals (hereinafter MPa). A much more recent publication, P. Shalek et al., "Hot-Pressed SiC Whisker/Si.sub.3 N.sub.4 Matrix Composites", 65 American Ceramic Society Bulletin 351 (1986), also utilized hot pressed silicon nitride powder with elongated "whiskers" of silicon carbide as reinforcement, but these whiskers still are no more than 0.5 mm in length. Use of silicon carbide whiskers in still other matrices is taught in U.S. Pat. No. 4,543,345 of Sep. 24, 1985 to Wei (alumina, mullite, or boron carbide matrices) and U.S. Pat. No. 4,463,058 of July 31, 1984 to Hood et al. (predominantly metal matrices).
A composite with oriented continuous fiber silicon carbide reinforcement was taught by Brennan et al. in U.S. Pat. No. 4,324,843. The matrix specified by Brennan was a crystalline ceramic prepared by heating a glassy, non-crystalline powder of the same chemical composition as the matrix desired in the composite. This description of the matrix appears to exclude silicon nitride, which was not taught in the patent as a matrix material. In fact, the broadest claim of this patent required a matrix of metal aluminosilicates or mixtures thereof. Perhaps for this reason, the highest modulus of rupture noted in this patent for any of its product was less than 100 MPa.
Still another microstructural variation for silicon nitride-silicon carbide composites was disclosed by Hatta et al. in U.S. Pat. No. 4,335,217. According to this teaching, neither fibers nor powder of silicon carbide or silicon nitride is used as an initial constituent of the composite. Instead, a powdery polymer containing both silicon and carbon is mixed with silicon metal powder, pressed, and then heated in a nitrogen atmosphere. The polymer gradually decomposes under heat to yield silicon carbide, while the silicon powder reacts with nitrogen to yield silicon nitride. The composition of the final composite is described as "comprising crystals of beta-silicon carbide, alpha-silicon nitride, and beta-silicon nitride . . . forming interwoven textures of beta-silicon carbide among said alpha-silicon nitride and beta-silicon nitride crystals without chemical bonding to provide micro gaps . . . for absorption of thermal stresses." The highest reported modulus of rupture for these composites was 265 MPa.
In this Hatta patent there was also a casual reference to "Conventional SiC-Si.sub.3 N.sub.4 composite systems . . . fabricated by firing a mixture of silicon powder with . . . SiC fibers in a nitrogen gas atmosphere at a temperature above 1220.degree. C." No further details about how to make such allegedly conventional composites were given in the specification, however.
Much of the non-patent literature in the field of silicon nitride-silicon carbide composites, which i general terms covers the same ground as the patents referenced above, was reviewed by Fischbach et al. in their final report to the Department of Energy under Grants ET-78-G-01-3320 and DE-FG-01-78-ET-13389. These investigators found that the types of fibers reported as very successfully used by Yajima in U.S. Pat. No. 4,158,687 were not satisfactory for their bonding because of a tendency for the interior of these fibers to debond from the sheath layer of the fibers during nitridation.
Metal coatings for ceramic reinforcing fibers are taught in U.S. Pat. No. 3,869,335 of Mar. 4, 1975 to Siefert. Such coatings are presumably effective because the ductility of metals allows absorption of the energy of propagating cracks by distortion of the metal. Composites with metal coated fibers are satisfactory for service at relatively low temperatures, but at elevated temperatures the metal coatings can melt and thereby seriously weaken the composite. The matrices taught by Siefert were glasses, which have lower temperature service capability than ceramics. Thus for ceramics, temperature limitation is a serious disadvantage for metal coatings on the reinforcement.
U.S. Application Ser. No. 700,246 filed Feb. 11, 1985 by Rice, now U.S. Pat. No. 4,642,271, the use of boron nitride as a coating for ceramic fibers to produce a crack deflection zone when the coated fibers are incorporated into composites. Silicon carbide, alumina, and graphite fibers and silica, silicon carbide, cordierite, mullite, and zirconia matrices are specifically taught. Results were highly variable. The toughness of composites of silicon carbide fibers in silica matrices was dramatically increased by a coating of about 0.1 micron of boron nitride, but the same type of coated fibers in zirconia or cordierite produced little improvement in composite toughness compared with composites of uncoated fibers.
A reason for the effectiveness of boron nitride was suggested, and there was additional relevant information, in European Patent Application No. 0 172 082 by Societe Europeenne de Propulsion, published Feb. 19, 1986. This teaches that boron nitride coated onto fibers by gas phase reactions between boron and nitrogen containing gases, as well as carbon coatings produced by certain kinds of pyrolysis, is deposited on fibers in laminar form, with relatively weak bonds between laminae. Thus a crack which enters the coating will normally have its direction of propagation changed if necessary so that the crack will propagate along an interface between laminae of the coating. These interfaces are parallel to the fiber surface, so that the crack is usually prevented from entering the fiber. Carbon and silicon carbide fibers in silicon carbide matrices are specifically taught by this application, and other matrices, such as alumina formed by decomposition of aluminum butylate, are suggested.
The advantages of a crack deflection zone around reinforcing fibers in a different matrix is illustrated by John J. Brennan, "Interfacial Characterization of Glass and Glass-Ceramic Matrix/NICALON SiC Fiber Composites", a paper presented at the Conference on Tailoring Multiphase and Composite Ceramics, held at Pennsylvania State University, July 17-19, 1985. This teaches that certain processing conditions lead to composites in which a carbon rich layer forms around SiC reinforcing fibers, and the carbon rich layer acts as a crack deflection zone. Similarly, advantages for boron nitride coatings on reinforcing fibers are taught by B. Bender et al., "Effect of Fiber Coatings and Composite Processing on Properties of Zirconia-Based Matrix SiC Fiber Composites", 65 American Ceramic Society Bulletin 363 (1986). As expected from the titles of these reports, silicon nitride is not taught as a matrix by either of them.
J. W. Lucek et al., "Stability of Continuous SiC (-O) Reinforcing Elements in Reaction Bonded Silicon Nitride Process Environments", Metal Matrix, Carbon, and Ceramic Matrix Composites, NASA Conference Publication #2406, p. 27-38 (1985), described silicon nitride matrices reinforced with silicon carbide fibers about 10-25 microns in diameter. These SiC fibers were derived from organo-silicon polymer starting materials. High strength, non-brittle composites were not achieved with these silicon carbide fibers. Lucek et al. reported, on the basis of information supplied by others, that some of the fibers they used had been precoated with boron nitride. Whether the fibers actually were coated has been subjected to some doubt since the original report. Lucek et al., because of government security restrictions imposed on them as a condition of the supply of the allegedly coated fibers, did not attempt to characterize the composites they had prepared to a sufficient extent to determine whether boron nitride, or any other material, actually was present around the polymer derived (PD) silicon carbide fibers they used after their composites had been made with the allegedly coated fibers.
The PD SiC fibers are known to be subject to some recrystallization, with accompanying volume shrinkage, and to partial volatilization, probably preceded by chemical reaction to give volatile products, upon heating in the temperature range required for formation of reaction bonded silicon nitride (RBSN). In contrast, chemically vapor deposited (CVD) silicon carbide fibers, also briefly studied by Lucek et al., are much less subject to detrimental changes while nitriding.
Lucek et al. determined that the tensile strength of their allegedly boron nitride-coated PD SiC fibers was degraded substantially less by exposure of the fibers to the temperature and atmosphere of nitriding than was the tensile strength of similar uncoated fibers. Nevertheless, they further determined, by flexural testing of composites made with various SiC fibers, that (1) the strengths of RBSN composites reinforced with both types of PD SiC fibers were substantially less than those of composites reinforced with CVD SiC fibers, (2) the strength of such composites made with the allegedly coated PD fibers was even less than that of similar composites with uncoated PD fibers, and (3) the tensile failure of the composites with both types of PD fibers was essentially catastrophic. Presumably some unascertained part of the process of making the fibers into composites destroyed and/or changed the properties of whatever coating was on them, so that when bonded into the RBSN matrix, the coating no longer functioned effectively for crack deflection.
At present, both CVD and PD silicon carbide fibers are very expensive, but it is believed that if significant volume demand developed, PD fibers could be made at much lower costs than CVD ones. There are also fundamental advantages to the smaller diameter of the PD fibers: smaller fibers are more flexible and versatile, especially in reinforcing complex shapes which require strength in more than one direction and which have thin sections. It is practically difficult to arrange a single thickness of fibers in a plane in an array which will give substantially isotropic reinforcement. It is therefore more common to use fibers within a single layer in nearly parallel array and to superimpose layers of such fibers with different orientations in order to obtain substantially isotropic mechanical properties. Obviously, if an object with a thickness little more than that of one layer of CVD fibers is desired, such an arrangement is impossible with such fibers, but it could be accomplished with the PD fibers, which can be obtained with less than one tenth the diameter of the CVD fibers. The smaller and more flexible PD fibers also can more easily be accommodated in sharp curves of the desired composite. On the other hand, the fundamentally greater thermal stability of the CVD type fibers should make their use safer in composites intended for sustained high temperature service.
For these reasons, it is advantageous to provide strong, tough RBSN composites with PD or other small diameter SiC fiber reinforcements as well as with larger fiber reinforcements, and composites of both types made be made according to this invention.
One generalization which appears clear from the background information recited above is that the properties of composites of silicon nitride and silicon carbide, like those of composites generally, are very sensitive to the details of microstructure of the composite. (A similar conclusion was stated in the Fischbach reference already cited.) Microstructural details in turn are sensitive to the chemical and physical characteristics of the starting materials and the processes used to convert the starting materials into a coherent composite body. Little predictability about the mechanical toughness of new and different composite microstructures has been possible heretofore.